Why opposites don't always attract

A lucky lab accident helps to explain the mystery of bouncing droplets.

It's a natural fact that opposites attract — or so scientists thought. But a new study of fluid droplets shows that opposites
can sometimes bounce right off one another. The results may seem esoteric, but they could have big implications for everything
from oil refining to microfluidic 'lab-on-a-chip' technologies.

The work, published today in Nature, began as a laboratory accident. William Ristenpart, a chemical engineer at the University of California at Davis, was studying
how the shape of a water column in oil changed as it was drawn towards an electrically charged plate. "I basically messed
up," he says. "I was applying a few kilovolts, the system shorted out and the water exploded."

Tiny droplets of water went ricocheting around the oil-filled chamber. But as Ristenpart watched, he noticed something odd:
oppositely charged water bubbles seemed to be bouncing off one another. "The first time I saw that I was terribly confused,"
Ristenpart says.

Charge puzzle

That's because, like other researchers, Ristenpart believed that oppositely charged water droplets would attract each other
and form larger drops. This property has long been exploited in the 'electrostatic separation' process used by the petroleum
industry to collect and remove bubbles of seawater from crude oil.

Ristenpart and his colleagues studied his laboratory accident for three years, and with the help of high-speed videos and
mathematical calculations they now claim to understand the phenomenon. Because of the force of surface tension, water droplets
are normally held in tight spheres. But as two electrically charged droplets come close to each other, the spheres begin to
warp — and at very short distances, a small bridge of fluid forms between the drops.

When the electrical charge is low, that bridge grows until the drops merge together, but when the charge is high, something
else happens: the bridge allows the droplets to exchange their charge and then snaps. The water flows back into the bubbles,
and by the time the two drops collide, they are back in their spherical shape. Rather than merging, their surface tension
causes them to bounce off one another like beach balls (see video).

Seeing is believing

"Wow, how can that be?" Frieder Mugele, a physicist at the University of Twente in Enschede, the Netherlands, remembers asking
himself on first seeing the result. But Mugele says he is wholly convinced by the group's explanation. "The fundamental principle
is captured by what they are saying," he says. "It's a very striking phenomenon."

A bigger question is whether the bouncing effect could actually be useful. Many scientists are working to develop microfluidic
systems — known as labs-on-a-chip — that can mix small amounts of chemical reagents or biological molecules. Electrical charge
is one way that chemicals can be moved around these chips, and the study's authors say that knowledge of the bouncing bubbles
could aid their development. Ristenpart says that the work could also find an application in the oil industry, which currently
uses building-sized electrostatic separators to remove seawater from crude oil. The American Chemical Society has given Ristenpart's
team a grant to see whether their research can create a more efficient separator, he says.

But even if the myriad potential applications don't pan out, Ristenpart is still planning a long future in droplet studies.
His group is now looking at unusual collisions in which the droplets break into a pair of daughter drops, one large and one
small. "That is not really well understood at all," he says. "There's a lot more thinking to do for sure."